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<!--[if gte mso 9]><xml> <w:WordDocument> <w:View>Normal</w:View> <w:Zoom>0</w:Zoom> <w:HyphenationZone>14</w:HyphenationZone> <w:PunctuationKerning/> <w:ValidateAgainstSchemas/> <w:SaveIfXMLInvalid>false</w:SaveIfXMLInvalid> <w:IgnoreMixedContent>false</w:IgnoreMixedContent> <w:AlwaysShowPlaceholderText>false</w:AlwaysShowPlaceholderText> <w:Compatibility> <w:BreakWrappedTables/> <w:SnapToGridInCell/> <w:WrapTextWithPunct/> <w:UseAsianBreakRules/> <w:DontGrowAutofit/> </w:Compatibility> <w:BrowserLevel>MicrosoftInternetExplorer4</w:BrowserLevel> </w:WordDocument> </xml><![endif]--><p class="MsoNormal" style="margin-bottom: 5.0pt; text-align: justify; mso-pagination: widow-orphan no-line-numbers;"><span style="font-size: 12.0pt; mso-bidi-font-size: 11.0pt; line-height: 115%; font-family: 'Times New Roman'; mso-ansi-language: EN-GB;" lang="EN-GB">Tropospheric ozone (O₃) is one of the most toxic compounds for plants in the atmosphere. </span><span style="font-size: 12.0pt; line-height: 115%; font-family: 'Times New Roman'; mso-ansi-language: EN-GB;" lang="EN-GB">The large amount of anthropogenic O₃ precursors in the urban areas promote</span><span style="font-size: 12.0pt; mso-bidi-font-size: 11.0pt; line-height: 115%; font-family: 'Times New Roman'; mso-ansi-language: EN-US;" lang="EN-US"> O₃ formation, thus making Mediterranean forests located in periurban areas particularly vulnerable to this pollutant. </span><span style="font-size: 12.0pt; line-height: 115%; font-family: 'Times New Roman'; mso-ansi-language: EN-GB;" lang="EN-GB">O₃</span><span style="font-size: 12.0pt; mso-bidi-font-size: 11.0pt; line-height: 115%; font-family: 'Times New Roman'; mso-ansi-language: EN-GB;" lang="EN-GB"> flux measurements have been carried out using the Eddy Covariance technique over a Holm oak forest located 25 Km from Rome downtown, inside the Presidential Estate of Castelporziano (Italy).Two transition periods - early Spring and late Fall - in two consecutive years were examined. The uncommon low precipitation recorded in both transition periods in 2012 allowed to evaluate the influence of water availability on </span><span style="font-size: 12.0pt; line-height: 115%; font-family: 'Times New Roman'; mso-ansi-language: EN-GB;" lang="EN-GB">O₃</span><span style="font-size: 12.0pt; mso-bidi-font-size: 11.0pt; line-height: 115%; font-family: 'Times New Roman'; mso-ansi-language: EN-GB;" lang="EN-GB"> fluxes during seasons which are not commonly affected by drought stress.Overall, </span><span style="font-size: 12.0pt; mso-bidi-font-size: 11.0pt; line-height: 115%; font-family: 'Times New Roman'; mso-ansi-language: EN-US;" lang="EN-US">the forest canopy showed to be a net sink of </span><span style="font-size: 12.0pt; line-height: 115%; font-family: 'Times New Roman'; mso-ansi-language: EN-GB;" lang="EN-GB">O₃</span><span style="font-size: 12.0pt; mso-bidi-font-size: 11.0pt; line-height: 115%; font-family: 'Times New Roman'; mso-ansi-language: EN-US;" lang="EN-US">, with </span><span style="font-size: 12.0pt; line-height: 115%; font-family: 'Times New Roman'; mso-ansi-language: EN-GB;" lang="EN-GB">peak values of mean daily O₃ fluxes of -8.9 nmol m^-2s^-1 at the beginning of flowering season and -4.6 nmol<span style="mso-spacerun: yes;">  </span>m^-2s^-1 at the end of Fall.</span></p><p class="MsoNormal" style="margin-bottom: 5.0pt; text-align: justify; mso-pagination: widow-orphan no-line-numbers;"><span style="font-size: 12.0pt; mso-bidi-font-size: 11.0pt; line-height: 115%; font-family: 'Times New Roman'; mso-ansi-language: EN-GB;" lang="EN-GB">O₃ fluxes were partitioned between stomatal and non stomatal sinks using the evaporative/resistive method based on canopy transpiration in analogy with an Ohm circuit. By comparison of the two years, water availability showed to be an important limiting factor during Spring, since in this season plants are more photosynthetically active and more sensitive to water availability, while in Fall, under conditions of low stomatal conductance,<span style="mso-spacerun: yes;">  </span>the dependence on water availability was less appreciated. </span></p><!--[if gte mso 9]><xml> <w:LatentStyles DefLockedState="false" LatentStyleCount="156"> </w:LatentStyles> </xml><![endif]--><!--[if !mso]><object classid="clsid:38481807-CA0E-42D2-BF39-B33AF135CC4D" id=ieooui></object> <style> st1\:*{behavior:url(#ieooui) } </style> <![endif]--><!--[if gte mso 10]> <style> /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Tabella normale"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;} </style> <![endif]-->